When an electroconductive plate is rotated between two fixed discs containing permanent magnets ("magnetic disc") arranged so that opposing magnets on the discs are of opposite polarity, eddy currents are generated in the rotating plate resulting in magnetic friction between the electroconductive plate and the magnetic discs. Such an arrangement incorporated as a resistance applying means on an exercise bike is disclosed in U.S. Pat. No. 4,826,150. The amount of the drag resulting from the magnetic friction in such a device may be varied by adjusting the relative positions of the magnetic discs between a position in which magnets of opposite polarity are positioned directly opposite one another (maximum magnetic friction) to a position in which magnets of like polarity are positioned directly opposite one another (no magnetic friction). Magnetic friction can also be varied by adjusting the air gaps between the electroconductive plate and the magnetic discs; increasing the gaps decreases the magnetic friction.
It is to be understood that the operation of a load applying device in which an electroconductive plate (copper plate, for example) is rotated relative to a magnetic disc, is different from the operation of a magnetic coupling device in which a ferrous plate is rotated relative to a magnetic disc in that in the latter instance there is a relatively strong axial attraction between the ferrous plate and the magnetic disc which is not present in the other instance between the electroconductive plate and the magnetic disc. It has been found that when a copper plate is rotated relative to a co-axial magnetic disc which is free to rotate and move axially, the magnetic disc will rotate in unison by magnetic friction with the copper plate and will also move axially toward the copper plate as the rotational speed builds up, but will not normally contact the copper plate. Ordinarily a small air gap at least about 3 mm. will be maintained. However, when the rotating plate is ferrous rather than copper, the magnetic plate will move directly into contact with the magnetic plate while stationary or rotating if permitted to do so. This operating distinction is significant in the operation of the present invention.
When a magnetic disc is free to rotate between and independently of a pair of electroconductive plates which are mounted for rotation on a rotary axis coaxial with the rotary axis of the magnetic disc, and the magnetic disc is driven, for example, relative to the electroconductive plates, the plates initially tend to rotationally lock to the disc, but, as previously mentioned, will not physically contact the disc even when free to move axially while rotating. It has been determined that after the electroconductive plates build up rotational speed equal to that of the magnetic disc, and the output shaft coupled to the plates is then abruptly stopped independently of the magnetic disc, the plates and magnetic disc will be axially repelled relative to one another. This is not true when ferrous plates are used instead of electroconductive plates.
In all instances herebefore known to applicant in which an electroconductive plate has been used in association with a magnetic disc for a coupling function, the plate has either been positioned between two magnetic discs as in the previously mentioned U.S. Pat. No. 4,826,150, or has been placed between a disc containing a permanent magnet and a yoke element engaging the disc so as to be magnetized. The latter arrangement is utilized in the speed governor disclosed in U.S. Pat. No. 4,826,150.
To applicant's knowledge the prior art has failed to recognize the advantages and improved efficiency to be gained in magnetic couplers by arranging a magnetic disc or discs between two electroconductive plates so that full utilization can be taken of both poles of the magnets in the discs. The present invention aims to provide improved couplers incorporating this superior arrangement.